"Bad Grandpa" is based on a character occasionally used in the "Jackass" television series: Irving Zisman (Johnny Knoxville), an eighty-six-year-old man who would perform some of the same dangerous stunts as his younger counterparts in order to see if onlookers would react differently to an octogenarian doing them. In "Bad Grandpa," Knoxville dons the ageing makeup once more, this time staying in character as the mischievous Irving for a whole feature-length film.

Irving's daughter has a son named Billy (Jackson Nicoll) whose chubby, cherubic face belies the fact that he is a bit of a troublemaker, even if he has a good heart. He has always had to fend for himself, because his mother is a drug addict who is finally busted by the police. Faced with the prospect of a long sentence, she asks Irving to take the boy across the country to live with his biological father. The old man reluctantly agrees, even though he doesn't seem to like kids all that much and doesn't have much of a relationship with his grandson. Before the road trip gets going in earnest, Irving's wife passes away, and he reacts with glee so intense it scares everyone around him. He throws is poor deceased wife's body in the trunk of the car and hits the road for what is sure to be an eventful trip.

From that point on, Irving morphs into the titular bad grandpa, drinking and smoking while Billy looks on. He also hits on anyone in a skirt as if he is a teenager with hormones running wild. Even in the midst of all this debauchery, Irving manages to bond with his grandson for the first time, revealing a new layer to the character. The two grow closer through a series of gags and stunts that were filmed live with unsuspecting people, much like the "Jackass" TV show and films. The road trip plot is combined effortlessly with the gags to make a movie unlike any other in the "Jackass" franchise.

It is a great tradition that once teenagers graduate high school and leave for college, they will inevitably do at least a few things that signal their newfound freedom. In "Bad Grandpa," Irving does this and a whole lot more. It is as if the death of his wife frees him from some unseen cage, and he acts out by drinking, smoking, gambling, and being hugely irresponsible. It's rare to see seniors doing things that a teenager would normally do, and it works pretty well here, creating the majority of the laughs in the film.

The "Jackass" television show was unlike anything else on television when it first came out. In fact, one could argue that it remains unique more than a decade after its first episode aired. It was such a huge success for MTV that the network decided to make several movies based on the show that were also successful at the box office. Even though the films have appealed to a fairly large and devoted audience, one of the criticisms of them has been that they lacked any kind of heart or plot. "Bad Grandpa" most definitely has a plot, and it actually has a lot of heart as well. Since "Bad Grandpa" is billed as being presented by "Jackass," it is ultimately a "Jackass" film, even though it veers wildly from the formula that made the earlier films so successful. This is actually a good thing, as it shows growth and a future for the franchise, which can now branch out into actual storytelling rather than just a series of increasingly dangerous stunts.

Devoted fans of the franchise shouldn't fret about these changes, however. "Bad Grandpa" still has plenty of stunts and high jinks to please those who have carried the series this far. Knoxville clearly knows who his core audience is, and does plenty to please them without breaking out of the grandpa character. He is helped tremendously by the fantastic aging makeup that he wears throughout the film, which looks extraordinarily real. If a forensic scientist were to sit down and try to sketch out what Knoxville would look like in about forty or fifty years, it would probably be remarkably close to how he looks in this film. This combination of the cosmetic prosthetics and the funny, occasionally touching plot make "Bad Grandpa" a cut above the usual "Jackass" film.